Noninvasive oximetry is based on spectrophotometric measurements of changes in the color of blood in peripheral tissues. The optical property of blood in the visible (between 500 and 700 nm) and in the near-infrared (between 700 and 1000 nm) region of the spectrum depends strongly on the amount of oxygen carried by the blood. Reduced hemoglobin, or deoxyhemoglobin (Hb), has a higher optical extinction, i.e. it absorbs more light, in the red region of the spectrum around 660 nm compared with oxygenated hemoglobin, or oxyhemoglobin (HbO2). On the other hand, in the near-infrared region of the spectrum around 940 nm, the optical absorption by Hb is lower compared to HbO2. The relative concentration of HbO2 in arterial blood is known as SaO2.
Noninvasive optical sensors for measuring arterial oxyhemoglobin saturation by a pulse oximeter (termed SpO2) are comprised of a pair of small and inexpensive light emitting diodes (LEDs) and a sensitive silicon photodetector. Typically, a red (R) LED centered on a peak emission wavelength around 660 nm and an infrared (IR) LED centered on a peak emission wavelength between 880 and 940 nm are used as light sources. In transmission pulse oximetry, the sensor is usually attached across a fingertip, foot, or earlobe such that the tissue is positioned between the light source and the photodetector. In reflection or backscatter mode pulse oximetry, the LEDs and photodetector are both mounted side-by-side on the same planar substrate. This arrangement allows measurements from multiple locations on the body where transmission measurements are not feasible.
Pulse oximetry relies on the detection of photoplethysmographic signal caused by variations in the relative quantity of arterial blood volume associated with periodic contraction and relaxation of the heart. The magnitude of this signal depends on the amount of blood ejected from the heart into the peripheral vascular bed with each systolic cycle, the optical absorption of the blood, absorption by skin and tissue components, and the specific wavelengths that are used to illuminate the tissue. The value of SpO2 is determined by computing the relative magnitudes of the R and IR photoplethysmograms. Electronic circuits inside the pulse oximeter separate the R and IR photoplethysmograms into their respective pulsatile (AC) and non-pulsatile (DC) signal components. An algorithm inside the pulse oximeter performs a mathematical normalization by which the time-varying AC signal at each wavelength is divided by the corresponding time-invariant DC component which results mainly from the light absorbed and scattered by the bloodless tissue, residual arterial blood when the heart is in diastole, venous blood and skin pigmentation. Since it is assumed that the AC portion results only from the pulsatile portion associated with the arterial blood component, this scaling process provides a normalized R/IR ratio which is highly dependent on SaO2 and is largely independent of the volume of arterial blood entering the tissue during systole, skin pigmentation, skin thickness and vascular structure. Hence, the instrument does not need to be re-calibrated for measurements on different patients. The empirical relationship between SaO2 and the normalized R/IR ratio measured by the sensor is programmed by the manufacturers into the pulse oximeter.
Noninvasive reflectance pulse oximetry has recently become an important new clinical technique with potential benefits in fetal and neonatal monitoring. The main reason for this application is the need to measure SaO2 from multiple convenient locations on the body (e.g. the head, torso, or upper limbs), where conventional transmission pulse oximetry cannot be used. Using reflectance oximetry to monitor SaO2 in the fetus during labor, where the only accessible location is the fetal cheek or scalp, provides additional convenient locations for sensor attachment.
While transmission and reflection pulse oximetry are based on similar spectrophotometric principles, it is widely known that reflection pulse oximetry is more challenging to perform and has unique problems. Therefore, practical solutions to problems associated with transmission pulse oximetry do not apply to problems associated with reflection pulse oximetry.
Reflection pulse oximetry can be adversely affected by strong ambient light generated for instance by light sources in the operating room or other light sources used for patient examination or phototherapeutic interventions. Commercially available reflectance sensors, such as the RS-10 sensor manufactured by Nellcor Puritan Bennett Corporation, are comprised of optical components imbedded in an optically opaque material to provide structural support and optical shielding from ambient lighting. Although some of the ambient light that is directed from the back of the probe in a predominantly perpendicular or oblique orientation is attenuated by the probe material, the highly sensitive photodetectors and high gain preamplifiers used in reflectance pulse oximetry tend to saturate even by indirect ambient light that can easily reach the photodetector by propagating through adjacent tissue structures in the vicinity of the probe. Furthermore, it is known that the relatively close proximity of the photodetector to the edge of the probe does not provide adequate protection against strong ambient lights. Therefore, the manufacture warns that using this sensor in the presence of bright lights may result in inaccurate measurements and further recommends covering the site with opaque material. Trying to provide additional protection from ambient light using for example a black cloth or optically opaque tape complicates the procedure considerably since it interferes with the monitoring procedure and also does not guarantee that the ambient light is blocked from reaching the photodetector.
Another practical problem in reflection pulse oximetry is the generally very weak pulsatile AC signals that are typically about 10 to 20 times smaller in amplitude compared to AC signals detected by transmission mode pulse oximeter sensors. Consequently, the normalized AC/DC ratios derived from the reflected R or IR photoplethysmograms that are used to compute SpO2 are very small and range from about 0.001 to 0.005 depending on sensor configuration or placement. In addition, the small amplitudes add considerable noise often leading to unstable readings, false alarms and inaccurate measurements of SpO2.
Most methods used to improve the accuracy of reflectance pulse oximeters aim at improving the signal-to-noise ratio by influencing the circulation underneath the sensor. For example, several investigators proposed improved sensors for application in reflectance pulse oximetry based on the application of skin heating to increase local blood perfusion [for example: Mendelson, Y. and Ochs, BD, “Noninvasive pulse oximetry utilizing skin reflectance photoplethysmography”, IEEE Transactions on Biomedical Engineering, vol. 35, no. 10, pp. 798-805 (1988), Mendelson, Y., Kent, J C, Yocum, B L and Birle, M J, “Design and evaluation of a new reflectance pulse oximeter sensor”, Medical Instrumentation, vol. 22, no. 4, pp. 167-173 (1988), Mendelson, Y., and McGinn, M J, “Skin reflectance pulse oximetry: in vivo measurements from the forearm and calf”, Journal of Clinical Monitoring, vol. 7, pp. 7-12, (1991), Takatani S, Davies, C, Sakakibara, N. et al. “Experimental and clinical evaluation of a noninvasive reflectance pulse oximeter sensor”, Journal of Clinical Monitoring, vol. 8, pp. 257-266 (1992), Konig, V, Huch R, and Huch A., “Reflectance pulse oximetry—principles and obstetric application in the Zurich system”, Journal of Clinical Monitoring, vol. 14, pp. 403-412 (1998)]. However, including means for heating the skin in order to increase local blood flow has practical limitations since they could cause burns to the skin especially in neonates which have very thin and sensitive skin.
Other methods, such as exerting moderate pressure on the probe [For example: Dassel, A C M, Graaff, R., Sikkema, M., Meijer, A., Zijlstra, W G, and Aamoudse, J G, “Reflectance pulse oximetry at the forehead improves by pressure on the probe”, J. Clinical Monitoring, vol. 11, pp. 237-244, (1995); Dassel, A C M, Graaff, R., Meijer, A., Zijlstra, W G, and Aarnoudse, J G, “Reflectance pulse oximetry at the forehead of newborns: The influence of varying pressure on the probe”, J. Clinical Monitoring, vol. 12, pp. 421-428, (1996)], have also been shown to be effective in increasing the pulsatile signals and thereby decreasing the R/IR variability. This was partly explained by an improved SNR due to stronger AC signals resulting from a better contact between the sensor and the skin. It was hypothesized that pressure on the sensor diminishes venous blood in the tissue underneath and, consequently, the disturbing influence of pulsating and non-pulsating venous blood is reduced considerably. Also, the relative increase in the change in vessel diameter during a pulse wave due to tissue pressure enhances absorption differences and flow velocities, resulting in an increased pulse size. Clinical measurements performed by the inventors confirmed that SpO2 measurements obtained using a reflectance sensor that was gently taped to the skin using a double-sided adhesive tape tend to be about 2-6% lower compared to measurements made by a standard transmission type pulse oximeter from the finger. For example, the inventors confirmed that by exerting a moderate amount of pressure on the sensor the readings are improved considerably. Accordingly, when the sensor is pressed against the skin, the readings of the reflectance pulse oximeter are increased and quickly coincide with the readings obtained by the transmission pulse oximeter. On the contrary, when the pressure on the sensor is removed, the readings by the reflectance pulse oximeter return to their initial baseline value and continue to fluctuate.
In all of the above references, other investigators failed to disclose practical means for incorporating an inexpensive, disposable and reproducible means for shielding the sensor from ambient light and simultaneously exerting adequate pressure on the probe. Improving the quality of the detected photoplethysmographic signals in reflectance pulse oximetry will be beneficial, since inaccuracies caused by noisy and weak pulsatile signals remain one of the major unsolved sources of errors in reflectance pulse oximetry.